The present system and method relate generally to hybrid motor vehicles, and more particularly, to a brake management system adapted to regenerate a fuel source of a hybrid motor vehicle.
Automobile manufacturers are continuously working to improve fuel efficiency in motor vehicles. Improvements in fuel efficiency are typically directed toward reducing weight, improving aerodynamics, and reducing power losses through the vehicle powertrain. However, the need to improve fuel efficiency is commonly offset by the need to provide enhanced comfort and convenience to the vehicle operator. As an example, manually-shifted transmissions are more fuel efficient than automatic transmissions due to lower parasitic losses. The higher losses associated with conventional automatic transmissions originate in the torque converter, the plate clutches and the hydraulic pump used to control operation of the hydraulic shift system. However, a vast majority of domestic motor vehicles, for example, are equipped with automatic transmissions due to the increased operator convenience they provide. Recent advances in power-operated shift systems have allowed development of “automated” versions of manual transmissions, which automatically shift between sequential gear ratios without any input from the vehicle operator. Thus, automated manual transmissions provide the convenience of a traditional automatic transmission with the efficiency of a manual transmission.
Passenger vehicle and heavy truck manufacturers are also actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrain systems equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, “hybrid” electric vehicles, which include an internal combustion engine and an electric or hydraulic motor, offer a compromise between vehicles powered by traditional internal combustion engines and full electric-powered vehicles. These hybrid vehicles are equipped with an internal combustion engine and an electric or hydraulic motor that can be operated independently or in combination to provide motive power to the vehicle.
There are two types of hybrid vehicles, namely, series hybrid and parallel hybrid vehicles. In a series hybrid vehicle, power is delivered to the wheels by the electric motor, which draws electrical energy from a generator or a battery. The engine is used in series hybrid vehicles to drive a generator that supplies power directly to the electric motor or charges the battery when the state of charge falls below a predetermined value. In parallel hybrid vehicles, the electric motor and the engine can be operated independently or in combination pursuant to the running conditions of the vehicle.
Improving the efficiency of hybrid vehicles includes recouping energy spent by the electric motor. Generally, the control strategy for recouping energy spent by the electric motor involves operating the motor in a reverse operation causing it to function as a generator during braking operations. However, attempting to recover spent energy through regenerative braking presents a number of issues. First, it is not possible or practical to recover all the braking energy at gross weights, at high speeds, or at high deceleration rates because some braking energy has to be transferred to the brakes under these situations. However, it would be desirable to retrieve as much of the braking energy as practical. Second, the amount of energy retrieved and transferred during regenerative braking varies depending on many variables, some of which will change during vehicle operation, such as the amount of wind acting on the vehicle, absorber motor torque variations, temperature changes, age of the vehicle, grade of the terrain, weight transfers, rolling resistance, etc. Third, the requested amount of braking (deceleration) by the driver will vary and usually increase as the vehicle slows. Fourth, regeneration will unbalance two-wheel-drive brake systems and could cause unusual brake and tire wear. Attempting to obtain the maximum amount of energy through regeneration generally unbalances the brake system of a two-wheel drive vehicle because regeneration provides a negative torque on the drive wheels, in addition to the braking force applied by the braking system. During regenerative braking, the non-drive wheels spin freely, only being acted upon by the braking system. This uneven application of negative torque between the drive and non-drive wheels results in a potential for skidding the drive wheels, reducing vehicle stability. With the loss of stability, moreover, there is typically uneven tire wear and brake wear. Accordingly, there exists a need for improved regenerative brake control systems for use in hybrid vehicles that facilitate an efficient, yet safe regeneration of energy.